Field of the Invention
The present invention relates to a method and to a system for monitoring movable objects in which each of the objects is provided with a transponder and in which a stationary node is arranged to communicate with the transponders, wherein the node sends to each transponder a signal which is identified solely by the transponder in question, wherein each transponder sends to the node a response signal in response to the signal from the node, wherein the response signal identifies the transponder concerned. The node can thus be forced to detect the speed and direction of each transponder towards and away from the node. Node associated means can then be caused to evaluate movement of the transponder in relation to the node, on the basis of the speed and direction of the transponder. This technique is known from U.S. Pat. No. 5,506,584, for instance.
The present invention finds use, for instance, in monitoring boats in harbours, and it is, of course, of interest in this regard to be able to limit the monitored area on the one hand, so that communication between node and transponder can always be maintained despite varying weather conditions. It is also of interest to be able to measure whether or not the boat/transponder moves relative to the node within the monitored area.
An object of the present invention is to provide a method and a system with which transponder movement and distance of the transponder from the node can be monitored, and with which the transponder can be tracked between different nodes.
This object is achieved with a method and system whereby each node is set up with a register of the transponders with which the node shall communicate; a plurality of separate nodes, each having such a register, are provided; and the nodes are enabled to communicate with one another such that a node is able to take over in its monitoring register a transponder from another node. The method and system may further include delivery by the node of an alarm signal when the distance of the transponder from the node deviates from a permitted value, or when the speed of the transponder exceeds a predetermined value. An alarm may also be triggered when a transponder leaves a node in which the transponder is registered. Detection by the node may be effected by the Doppler effect.
The fact that the transponder delivers a unique signal enables the unique transponder signal to be associated with a telephone network subscriber. For instance, by coupling the node to a telephony system, the subscriber is able to send a message to the transponder via the telephony system when the node discovers that the transponder concerned is located outside its permitted area or moves at a speed which exceeds a predetermined value. For instance, the node can be caused to detect a Doppler-shift response from each transponder, this Doppler shift constituting a measurement of the speed at which the transponder moves towards or away from the node.
The transponders normally respond to a coded microwave signal from the node with a coded Doppler-shift response. The node sends coded signals to the transponders in accordance with a given sequence at pre-programmed time intervals, preferably constant time intervals. The signal from the node is coded so that only the intended transponder is able to react to the node signal. The transponder includes a computer which is programmed to activate the transponder for transmission when the code of the node signal agrees with the identity of the transponder. When the transponder returns a coded signal to the node, this reply signal is also given a code that identifies this transponder. Naturally, the computer in the transponder may also be programmed to add additional information to the transponder output signal. For instance, a theft alarm or burglar alarm may be coupled to the computer of the transponder, so as to cause the computer to add corresponding information to the transponder reply signal, thereby enabling an alarm signal to be sent by the node to an alarm receiver, via a communications system (telephony system). For instance, the transponder may have an identity corresponding to a telephone subscriber number, so as to enable an alarm to be signalled to the subscriber with a message corresponding to the current alarm state associated with the transponder in question, this signal being sent directly from the node via a telephony system, for instance the GSM system.
The technique of detecting movement of the transponder towards or away from the node with the aid of a Doppler effect for instance, also enables the absolute position of the transponder to be calculated by numerical integration of the speed fixes, although absolute determination of the transponder position will preferably be carried out at close intervals. However, the invention is not limited to the use of a Doppler effect for detecting transponder movement and speed.
The invention enables movement of the transponder to be detected, or sensed, within the permitted transponder area that lies well within the range of the node/transponder, therewith giving a clear indication that the transponder is moving away from its expected position while the transponder is located within the communication range of its node.
The fundamental concept of the invention enables several nodes to be used, each of which monitors its respective group of transponders, said nodes communicating with one another. When a transponder moves away from its node, out of the permitted area, the node is thus able to communicate with nearby nodes and initiate the nodes to search for the transponder concerned, the code of which is then passed to these remaining nodes. When one of these remaining nodes receives a response from the transponder concerned, the transponder can be dismissed from the groups of objects monitored by the other nodes. For example, the nodes may lie at a distance of 3 km apart, to enable such a handover to take place. The nodes may in turn, communicate with a main node which monitors the permitted movement area or a given transponder. The area can then be represented by a specific group of nodes. It is thus fully possible to connect the main node to a computer that monitors the permitted area of a given transponder. For instance, the main node may allow a given transponder to move within the permitted areas corresponding to overlapping of the permitted transponder areas of a predetermined number of nodes, the main node otherwise triggering an alarm to the person responsible for the transponder concerned. For example, a yacht may have been hired with under the provision that it must not leave an area defined by pre-determined permitted, mutually overlapping communication areas of the nodes with the transponder, wherewith the main node is able to initiate an alarm of some kind if the transponder concerned disappears from the permitted area. The main node may, in turn, be connected to other main nodes via known telecommunications equipment.
The nodes may be comprised of mobile units that have an own GPS equipment that determines the location of the node. The node may, in principle, include a transmitter, a receiver and logic that enables the node to monitor the transponders that are in the area monitored by the node and registered therein.